The present invention relates to a rotary head assembly for VTR, video tape recorder, in which a rotary cylinder must be rotated at high speeds with a higher degree of accuracy.
The most important function of the rotary head cylinders for VTR is to maintain correct speed and position relationship between a head and a tape and to reproduce this correct relationship at any time.
When the relative speed between the head and tape varies, the variations in time axis of reproduced signal result so that variations in color in the pictures and horizontal slipping of the picture occur. Therefore such variations must be minimized. More particularly, the rotary cylinder must be free from variations in rotational speed and from radial or lateral vibrations as much as possible.
In the prior art VTR head assemblies, the ball bearings are used to support the shaft of the rotary cylinder which carries the head so that the following problems arise. [1] Variations in torque are very sensitive to the volume of lubricant, for instance grease, and to the conditions under which the lubricant is sealed. When grease is sealed in a large quantity, the load varies over a wide range. On the other hand, when the quantity of sealed grease is small, wear accelerates. In addition, variations in torque are also caused by the undulations of the surfaces of the races of the ball bearings.
The recent trend of VTR rotary head assemblies is towards compactness. As a consequence the inertia effects of the rotary cylinder are reduced so that the variations in rotational speed tend to occur more frequently. [2] Vibration is caused mainly from poor surface finish of the balls and outer and inner races of ball bearings, elastic vibrations of the outer and inner races and gaps between mating parts. Radial or lateral vibrations cause the deviation of the axis of rotation of the rotary cylinder so that variations in rotational speed result and consequently complex and adverse effects on the reproduced pictures occur.
The above-described problems arise inevitably as far as the ball bearings are used. To overcome these problems, extensive studies and experiments have been made. For instance, in order to minimize the vibrations of the rotary cylinder, the dimensional accuracies of the ball bearings, housing and other parts are increased as much as possible, and the moment of inertia of the rotary shaft is increased as much as possible. However, because of the tendency to compactness in size of VTR, the rotary cylinders are also reduced in size so that the flywheel effects are reduced accordingly. As a result the vibrations of the ball bearings become more pronounced.